Google Stackdriver Error Reporting - Aggregates and displays errors produced by cloud services
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Million.js - A fast, <1kb virtual DOM
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
Sentry.io - From error tracking to performance monitoring, developers can see what actually matters, solve quicker, and learn continuously about their applications - from the frontend to the backend.
Better Uptime - We call you when your website goes down